Magnesium oxide hot process for silica removal from water



present,

Patented Jan. 5, 1943 UNITED STATES PATENT MAGNESIUM oxma SILICA REMOVAL Charles A.

Philadelp Pa a H. Betz and Noll and John J. phia, raiigssiznors to W.

L. Drew Betz OFFICE HOT PROCESS FOR FROM'WATER Maguire, Philadel- H. & L. D; Betz, composed of William No Drawing. Application September 6, 1939,

Serial No. 293,570

3 Claims. (Cl. 210-23) .to 9. Prochereinbefore referred to.

In order to eliminate the silica from'natural.

water, various methods have heretofore been devised, but heretofore with only partial commercial success, due to the introduction of high dissolvfed solids in ferrous, ferric and zinc hydroxides; and similar reagents. However, after many years of research upon the subject, none of them offers results approaching those hereinafter described,

and as illustrated by-the tables incorporated herein.

dependently thereof.

More specifically;v the preferred form of the by test are found ,to similarly function with magnesium oxide or magnesium carbonate.

REMOVAL OF SILICA AS THE RESULT OF VARYING PROPORTIONS OF MAGNESIUM OXIDE In order to illustrate the results obtained by proportions of magnesium the use of magnesium oxide Conditions: TABLE I 1 liter samples of water with a silica concentration of 42 P. P. M., as SiOa Temperature 95 C. 15 minutes retention and stirring time 40 P. P. M. sodium hydroxide added SiOs Percentage removed Bio Magnesium oxide added grams removed P. P. M.

eases it will "be noted that with an From .this table, initial silica concentration in the water of 42 P. P. M. at a temperature of 95 0., with 40 P. P. M. sodium hydroxide added and 15 minutes retention and stirring time, a mere 0.1 gram magnesium oxide, equivalent to 100 P. P. M., reduced the silica to 18 P. P. M., while an increase in the magnesium oxide to 0.3 gram effected a further reduction in the silica to only 0.1 P. P. M., or in other words a removal eillciency of 9.9.%

Eflect of temperature It has been found that an increase .in temperature results in an increase in the efficiency of the reaction, and in the consequent removal oi silica from water by means of magnesium oxide in the presence of a fixed proportion of sodium hydroxide, as indicated by the accompanying Table II:

15 minutes retention and stirring time 0.1 gram magnesium oxide added 40 P. P. M. sodium hydroxide added 1 liter samples of water 1 P alkalinity is the titratable alkalinity to the phenolphthalein colorless end int H 8.3;.

I M alkalinity is the t tratab e alka inity to the methyl orange salmon colored end poin (pH 4.3).

Analysis of treated water in P. P. M.

Temperature centigrade BL M 5mm 5. 53.58 e a m e is Ca 0: Ca 0:

From this table it will be noted that with a relatively low water temperature, such as that which is found in an ordinary boilerroom, the initial silica content of 22 P. P. M. with 0.1 gram magnesium oxide and 40 P. P. M. sodium hydroxide, under 15 minutes retention and stirring time, is reduced only to 16 P. P. M. As the temperature is increased, while all other factors remain the same, it will be seen that at 95 C., the silica was reduced to a mere 1 P. P. M. It is also interesting to note that with an increase in temperature, other factors being equal, the hardness of the water as calcium carbonate is decreased from 88 to 28 P. P. M., while the alkalinity is similarly reduced, both of which factors relatspecifically, with no sodium duced from 22 P. P.

ing to hardness and alkalinity are of prime importance in the conditioning of water for industrial purposes. As the ,temperature of approximately C., is substantially thesame as that which is ordinarily maintained in hot-process water softeners, it will be apparent that a high degree of efliciency of the process can be expected, when using the same for silica removal in such softeners concurrently with the so-called lime-soda process. It should also be understood that, when mention is hereinafter made to 95 C., such expression is intended to represent the entire temperature range at which the process operates adiacent to the boiling point and as low as approximately 75 C.

Additional tests were made with temperatures of 23 C., but at such low temperature, it was found impossible to obtain results in any way approximating those obtained with the higher tem peratures noted in Table 11, even when increasing the magnesium oxide to 0.3 gram and the stirring and retention time to as much as 180 minutes (3 hours), as compared with the 15 minutes retention and stirring time used in each of the comparative tests shown in Table 11. In fact, with a temperature as low as 23 C., and 15 minutes retention and stirring time, the silica content is reduced from 22 P. P. M only ,to 19 EFFEC OF PROPORTIONS OF SODIUM HYDROXIDE From the accompanying Table III, the results will be apparent when using various proportions of sodium hydroxide with a substantially fixed proportion of magnesium oxide, while this table also indicates the definite need for controlling the alkalinity of the water. To illustrate more hydroxide, but 0.2 gram magnesium oxide added, the silica is re- M. to 16 P. P. M., while upon using substantially the same proportion of mailnesium oxide, i. e., 0.3 gram, but adding 20 P. P. M., sodium hydroxide, there is a sharp diminution in the silica toa mere 1.5 P. P. M., and this is only slightly further reduced to 1.0 P. P. M. by doubling the quantity of sodium hydroxide. However, while the alkalinity has thus far been maintained within reasonable limits, an increase of sodium hydroxide to P. P. M. effects a considerable-decrease in the hardness of the water, but with the sharp increase in alkalinity there is a pronounced decrease in the efllciencyof the silica removal, so that there is under such conditions 3.0 P. P. M. remaining silica, thereby showing that too much sodium hydroxide tends to retard'or inhibit the full action otherwise of the magnesium oxide. It should be added that by increasing the sodium hydroxide to the neighborhood of 40 P. P. M., and .the magnesium oxide to approximately 0.3 gram, this process also serves in lieu of the lime-soda process, while simultaneously operating at high efilciency in the removal of silica. The facts illustrated by Table 111 further indicate very definitely the need for conwater following the 100 P. P. M.

Table IV.

P. P. M., and the be noted that when using no sodium hydroxide,

the hardness of. the water was immediately increased from 40 P. P. M. to 76 P. P. M., while presence of substantial quantities of sodium hydroxide reduced the hardness below that of the water in its original condition.

' hardness to 32 P. P. M.,

TABLE 111 Analysis of original sample:

1 P. P. M. Hardness as CaCOa P alkalinity as CaCOs 0 M alkalinity as 08.00:: 28 Silica as SiO2 22 Conditions:

3 liter samples of raw water Temperature-95 C. 15 minutes stirring and retention time sodium Magne Analysis oi treated water fi 521 3: PAlka MAlka H d P. P. M. added 33%? 151 5 16230? g g 'g P. P. M. P. P. M. P. P. M. P. .P. M.

76 20 4s 1s 56 a2 04 1.5 34 4o 12 1.0 1o 84 124 3.0

USE OF THE PROCESS WITH HOT LIME- SODA S OFIENING hardness Team: IV Effect of magnesium oxide in conjunction with softening Conditions:

1 liter samples of water; Temperature-95 C.

removal of silica in hot process lime and soda v 3 and these results remain the same, even by increasing the retention time from 15 minutes to minutes. By adding first M.,and then 40 P. P. M. of sodium hysilica removal was consistent with the results shown in oxide to 0.15 gram, minutes retention Control of the present process is -of paramount importance as is clearly shown by an examination accompanying tables, for ii the process is within fairly critical a substantial addition to the solid content water will be eflected, and this, as is wellknown; is undesirable, especially in the treatment of boiler i'eedwater, and gree in other types of The some tables .also chemical balance must be maintained in the as otherwise increased hardness of the treated water will almost invariably result,

only to a slightly less de- The factor of temperature of approximately 0., in addition From this table, it will be notedthat the orig- Teat Test Test Test Test No.8 No.4 No.6 No.6 No. 7

Hardness as 01100: in P.P.M... m 2) 6 34 34 P alkalinity as 0:100: in P.P.M 52 32 62 24 22 M alkalinit as 08003 in P.P.M 84 82 96 60 Si SilicaasSi |1nP.P.M 2.6 2.0 3.0 -0.0 0.0 to to Magnesium oxide in grams 0. 1 0. 1 0.! 0. l 0. 1 0. l5 0. 15- Sodmmhydroxide in P.P.M a0 20 40 Retention time in minutes l6 15 60 15 60 i6 15 60 1 Hot process lime and soda treatment alone. I inal hardness as calcium was sharply reduced to but 1.0 P. P. M., and the," chemical mixing currently tends to alkalinity, with the further dedecrease'in the solids content.

It has furthermore been found that ,as a matter of control it is necessary in order to obtain proper efflciency of silica removal to proportion the magnesium dry to the water to be treated. If masnesium oxide is simply mixed into a slurry in a tank with water, it hydrolyzesto was lessened,v

III. In the last two columns oi Table IV further increased water for industrial purshow that a proper decrease the hardness of the.

magnesium hydroxide. Once' 'in this form, if then added from the chemical mixing tank to the -water to be treated, silica removal will not be effected, since magnesium hydroxide formed externally and added to water will not remove silica.

- It has been found that the addition of magnesium oxide to water in a chemical mixing tank requires approximately one-half hour for the hydrolysis, so that if the magnesium oxide remains in contact with the water in the chemical mixing tank for a period of time greater than one-half hour,- and the mixture is then fed to the water to be treated, a proper efiiciency for silica removal will not be obtained.

Magnesium oxide can not be mixed into water with an alkali such as calcium oxide, sodium-hydroxide, or sodium carbonate, without the immediate precipitation of magnesium hydroxide, and magnesium hydroxide which is formed externally to the water to be treated is, as herein noted, in-

magnesia obtained from similar sources.

eillcient for the removal of silica. There is nosuch delay of one-half hour in the formation of magnesiumhydroxide, when the magnesium oxide is added to the water in the presence of an alkali, since the magnesium hydroxide forms immediately.

In order, ther fore, to obtain precipitation of magnesium hydroxide in situ, it is necessary that the magnesium oxide be proportioned dry, and fed dry either to the water to be treated, or into a stream of water where a retention time of less than one-half hour has been provided, before introduction by means of such stream into the water to be actually treated for silica removal.

EFFICIENCY OF VARIOUS FORMS OF MAGNESIUM OXIDE By comparing magnesium oxide from various sources, it has been found that there is a wide difference in the physical as well as the chemical characteristics of the magnesia and magnesite, as shown by the accompanying Table V:

Team: V

Turbidity Lbs/cu. it. as S102 P.P.M.

U. S. P. light magnesium oxide V 16.0 Commercial magnesia (source A). 17. 7 216 Commercial magnesia (source B) 21. 2 200 Technical magnesium oxide 24. 7 100 1 Commercial magnesia (source C)... 27.8 144 Commercial magnesia (source D) 29. 3 134 U. B. P. heavy magnesium 0 de.. 33.0 200 l Calcincd magnesite (source E)... 45. 5 63 Calcined magnesite source 46.00 '32 8 Calcined magnesite (source 51. 5 17 Sources A, B, C and D refer to (our different samples of commercial magnesium oxide obtained from seawater bitterns, and produced by the use of quick-lime for the precipitation of magnesi Sources E, F and G refer to three different samples of commercial magnesite obtained through the calcining of mined magnesite.

Referring to this table, in which magnesium oxidels listed according to various types and sources, the comparison shown is based upon the respective weights of the samples used, and it will be noted that these range all the way from 16.1 lbs. per cu. ft. for U. S. P. light magnesium oxide to 51.5 lbs. per cu. ft. for calcined magnesite from source C. Thus, we havev practically pure magnesium oxide of a relatively light and fiuffy nature at one end of the scale, while a relatively low grade of heavy weight and compactness is at the other end. In substantially the same ratio as that indicated by their respective weights, the several types of magnesium oxide are efllcient I Turbidity determinations as ment'of properties are known to increase with the degree forms are similarly least eflicient.

the U. S. P. light magnesium oxide when used in the present process. the purer and lighter weight forms producing the highest efliciency, while the least pure relatively heavy However, as the cost of magnesium oxide is to a large degree a controlling factor in silica removal for industrial purposes, it is not generally considered commercially feasible to'use because of its relatively high cost, especially when the so-called commercial and but slightly less pure magnesia is considerably less expensive, and operates with almost exactly the same degree of efllciency.

The relative values of various forms of magnesium oxide for silica removal is also generally indicated by the degree of turbidity, while this characteristic also oflers a meansfor comparing For example, the measurement of turbidity, as indicated in Table V, enables one to determine that form of magnesia best suited for this work irom'similar as well as from difierent means of manufacture. here illustrated are measured on a 0.025% solution of magnesia with distilled water, after stirring and permitting two minutes time to elapse for settling. The results are expressed on the standard scale for measureturbldity as SiOz in P. P. M. Adsorptive of particle subdivision, therefore, with a smaller particle size and consequently greater turbidity, as measured above, one can expect'greater adsorptive properties generally in the removal of dissolved silica from the solution.

CONIPARATIVE DATA ON COMMERCIAL FACTORS INVOLVED TABLE VI 1.2 Test 1 Tcst2 Test 3 Test 4 Test 5 Test 6 Hardness as CaCO;

m.. 16 300 64 Sulfate u 884 p. 4 490 496 3 P alkalin ty as 2 CaC0i ..p. p. m.. 30 78 26 4 M alkalinity as CaCOs---p. p. m.. 78 182 110 92 11 9. s 10. 0 s. o 9. 0 Silica as BiOz p. .m 3.0 1.0 2.5 16 Magnesium oxide p. p. 111.. 300 Sodium hydroxide 426 p. 30 200 300 Magnesium sufiate p. p. m.. B00 800 Ferric suliate Test l-Using U. S. P.

I light magnesium oxide. Reaction and stirring time 15 minutes. Tergprilfi" C. Test 2-Usmg oommerc magnesia (source A). Conditions as in test 1. Test 3Using commercial magnesia (source B). Conditions as in Te fii -Using magnesium sulfate. Conditions as in test 1.

Test 5Using magnesium sulfate. Conditions as in test 1.

Test 6-Using ferric sulfate. Retention and stirring time 60 minutes.

Temp. 25 C.

Table VI has been prepared in order to graphically illustrate advantages possessed by the present magnesium oxide process, as comparedwith other processes, which to the uninitiated might appear to be closely allied therewith, if not actually the equivalent thereof. For all of the six tests run and indicated in this table, the same raw natural water was used and the characteristics of the same are first shown as characterizing a hardness as calcium carbonate of '74 P. P. M., of sulphate as 4 P. P. M. and silica as 56 RP, M.

1, 2, and 3, '300 P. P. M. magnethe treatment of water for sium oxide and 30 P. P. M. sodium hydroxide were used together, test from 70 P. P. M; to 110 P. P. M.

In test 2, the removal initial 56 P. P. M. to 3.0 P. P. M., while the hardincreased only from the initial 70 P. P. M. to 78 P. P. M. In test 3, the removal of silica was from an initial 56 P. P. M. to 2.5 P. P. M., while the hardness of the water was decreased from the initial 74 P. P. M. to 66 P. P. M., which is substantially the same as that of test 2, while the total alkalinity was increased by only 2 P. P. M.,i'rom the initial-70 P. P. M. to 72 P. P. M.

Tests 4 and 5 were based upon. the use of 800 P. P. M. of magnesium sulphate with #126 P. P. M.

last mentioned test the hardness jumped to the was eil'ected, especially in test 4, when it was retions by means 7 Thus, for example, the quantity of magnesium that is required to ac- [MgtOHhhiMgSiOahiHgO]: Some or the data appears to indicate that the is obtained which points to the inescapable. conof using ferric sulphate (460 P. P. M.) with so- 16 P. P. M., as compared with the much lower results noted 'were attained only by a retention minor commercial value, especially in the treat- 4 creased from 70 RP. M. to 92 P. P. M., and the 50 v v silica, redu d from th init 5 P P M, only t ployed is not in a constant ratio to the amount of net remaining quantity oi silica resulting from tests 1 to 5 inclusive, and in addition, even the suits noted, as ferric sulphate cannot be eilective-- 6o paramount importance is the. fact that, in order to obtain eiiective removal of silica from solution Thus, the process represented crease in sulphate above mentioned. U E OF MA NEBIUM CARBONATE v monnmsm OF REACTION From the chemical standpoint, it is interesting the reaction proceeds quite similarly to that when to note that the precipitation of silica from solu- 7 using magnesium oxide. The magnesium carbo nate can be used either in a mercial material is usually employed in a slurry form. in slurry form, due to commercial magnesium cardry form as the com purchased, or can be However, there is an bonate frequently being relatively higher in price,

and also the fact that it may be precipitated from any cheap magnesium salt to produce the slurry form, as for instance from magnesium chloride suiphatewith sodium carbonate as washing is completed, dry this precipitate, but instead it may be permitted to remain in the slurry form. while this form of magnesium carbonate also offers the advantage of effecting a greater degree of silica removal than does magnesium carbonate in dry form, as indicated by the accompanying Table VII.

TAanaVII j Analysis of or'iginalsample:

P. P. M.

using magnesium carbonate in; a

aeoaaee softening process removed from Hardness as CaCOs 136 P alkalinity as CaCOa. 24

Malk8-linityasCaCOs 21 Silica as SiOa ....i

Conditions:

- 1.0 liter samples 01 water minutes stirring and retention time Temperature-45 C. Magnesium carbonate added with lime and soda ash LIME AND SODA'TRLEA'I'MENT MAGNESIUM CABBONATE ADDED IN swims man 20 P. P. M. and 100 P. P. M., and the P alkalinity Analysis 0! treated water, Addi- Grams, in d Silica ling Hard- Palka-Malka- W bub CaO nesa aslini aslini as g' 0500. Ca o. Ca 0. f

P.P.M. RPM. P.P.M. P.P.M. 40 a2 92 1s a 24 102 0 l5 24 100 4 17 88 3 l8 (MAGNESIlIli/LCARBONATE ADDED IN DRY roam 6.i cu m ea 84 a 12 0.2. 135.0 .12 to 84 m n 0.5.. 2025 20 to as n 10 Thus, magnesium carbonate can be used in all ofthe applications in which magnesium oxide is of advantage in conjunction with the hot-process lime-soda water softening. However, although as before stated, magnesium carbonate serves in M general as quite a satisfactory substitute for magnesium oxide in the removal of silica from tlon, magnesium oxide in the forms shown in tests 2 and 3 of Table V1 is somewhat more eflleient (not mentioning testl of Table VI, dueto the much higher cost of 13.8. P. light magnesium oxide.) Also, when combining the hot lime-soda soluuse or magnesium the water to between SUMMAR The net results of the research work, represented by the accompanying tables and the foregoing description, and considering all of the factors involved, is that silica-is most economically solution for industrial purposes, in boiler water treatment, by the oxide in the form of the relatively purer and lighter weight forms of commercial magnesia; that this process functions best at high temperatures as for instance at approximately 95 C., and sols eminently well adapted for use concurrently with the lime-soda water softening process; best carried out with the addition of sodium hydroxideor' its equivalent; .and that fairly close approximation of the results thus obtained are similarly achieved by the use of the slightly less economical magnesium carbonate.

Having thus described our invention, what we employ additional and especially claim as new and desire to protect by Letters.

Patent of the United States is:

1. The method of removing from natural water, ch consists in heatin boiling point, and admixing 2.4 parts to '1 :5 parts of light weight magnesium oxide per part of initial silica expressed as water, and sufficient sodium hydroxide to produce a total M alkalinity of thetreated water between 15% and of the M alkalinity and then separating the precipitate from the water.

2. The method of removing dissolved silica irom natural water, which consists in heating the water to the neighborhood of C. and admixing therewith approximately 2.4 parts to 7.5 parts light weight, substantially pure magnesium oxide per part of initial silica expressed as SiO: in the water, and duce a total M alkalinity of the treated water between 20 P. P. M. and P. P. LL, andthe P alkalinity being between 15% and 85% of the M alkalinity and then separating the precipitate (rem the water.

3. The method of removing dissolved silicai'rom natural water while in the hot lime-soda softening process, which. consists in admixing therewith approximately 2.4 parts to 7.5 parts lightweight magnesium oxide per part of initial silica expressed as separating the precipitates fromthe water.

v a BABIES A. NOLL. JOHN J. MAGUIRE.

lime for a main-- balance of lime-soda that the magnesia process is dissolved silica 810: in the water and then 

